Imide compounds such as N-hydroxyimide compounds, N-alkoxyimide compounds, and N-acyloxyimide compounds are known to act as very excellent catalysts in oxidation of organic compounds using molecular oxygen, since they show high reaction efficiencies and high selectivities and can be applied to a wide variety of substrates in the oxidation (e.g., Patent Documents 1, 2 and 3). However, these imide compounds undergo denaturation and thereby gradually decrease in activity in reaction. The denaturation may occur through several processes, and main possible factors thereof are as follows.
The imide compounds change into N-hydroxyimides before they develop activities in the reaction. A possible mechanism for developing the activity is as follows. Initially, oxygen atom and hydrogen atom of the hydroxyl group adjacent to the nitrogen atom of imide easily undergo homogenous dissociation to form a radical. The formed radical withdraws hydrogen from a substrate to be oxidized to initiate a radical chain of oxidation reaction. The catalyst imide compound returns to its original configuration, repeats the same operation, and elongates the radical chain. However, the radical formed from the N-hydroxyimide inevitably induces, due to its own nature, a “termination reaction” in which the radical is bound typically to a radical formed from the substrate to be oxidized. As a result, the N-hydroxyimide structure serving as an active site of the N-hydroxyimide is lost. The resulting compound derived from N-hydroxyimide and formed in the termination reaction further undergoes denaturation without returning to the N-hydroxyimide.
In another possible denaturation process, the imide moieties of imide compounds may be hydrolyzed by water formed in oxidation reactions and thereby undergo denaturation. In yet another possible denaturation process, N—O bond (nitrogen-oxygen bond) of imide compounds may be cleaved typically by the action of a coexistent metal promoter (co-catalyst). In any case, when the imide compounds are reused, losses thereof due to denaturation in the course of reaction must be made up for regardless of the reaction system such as batch system or continuous system, and this invites increased cost.
As possible solutions to these problems, Patent Documents 4 and 5 each disclose a technique of recovering a denatured product (decomposed product) of a N-hydroxyimide compound, subjecting the same to a certain treatment according to necessity, reacting the treated product with hydroxylamine to reconstruct a N-hydroxyimide structure as an active site to thereby regenerate the N-hydroxyimide compound, and using the regenerated compound in oxidation. This technique is economically advantageous as compared with the cases where a fresh N-hydroxyimide compound is purchased or the compound is produced after purchasing materials for the compound. However, isolation of denaturated N-hydroxyimide compounds requires much energy, since the amounts of such N-hydroxyimide compounds used in oxidation of organic compounds with molecular oxygen are generally very small. Additionally, it is difficult to fully make up for losses due to denaturation by regenerating the catalysts from the isolated N-hydroxyimide compounds.
Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 08-38909,
Patent Document 2: Japanese Unexamined Patent Application Publication (JP-A) No. 10-57814,
Patent Document 3: Japanese Unexamined Patent Application Publication (JP-A) No. 09-327626,
Patent Document 4: Japanese Unexamined Patent Application Publication (JP-A) No. 11-188265,
Patent Document 5: Japanese Unexamined Patent Application Publication (JP-A) No. 2001-286765.